The optical behavior of silicon photonic modulators and related devices can be controlled using the change in refractive index produced as a result of a change in electronic carrier concentration. A device offering control of that kind will typically include an optical waveguide in which doped regions, such as p-type and n-type regions, are disposed so as to form a diode. A bias voltage applied across the diode can be varied so as to vary the carrier concentration in the diode junction region, thereby causing the refractive index to change.
The above-described effect can be utilized in various ways. For example, a change in optical path length due to the index change can modulate the output from an interferometer by shifting the relative phase between two interferometer arms between 0 and 180°.
However, the index-shifting effect is relatively weak. In practical applications that are meant to work at low power, it is therefore advantageous to enhance the effect through optical resonance. That is, the shifting of an optical cavity into and out of resonance with an optical field can cause very large changes in the interaction of the cavity with the field. Small changes in the optical path length within the cavity, brought about by changing the bias voltage, may be sufficient to shift a particular wavelength from a condition of being on-resonance to a condition of being off-resonance or vice versa. This may, for example, cause relatively large changes in the optical transmission of a bus waveguide coupled to the cavity.
Optical cavities, exemplarily in the form of microring and microdisk resonators, have been made and have been usefully employed in the manner described above. However, the resonant wavelengths of these and similar devices are very sensitive to temperature and manufacturing variations. It was therefore recognized that for such devices to be generally useful, means should be provided to tune individual devices to the desired operating wavelength.
In one approach to wavelength tuning, a metal strip heater is added directly above the resonator and electrically isolated from the resonator by a layer of silicon oxide. Such an approach is relatively inefficient, however, because the oxide layer is typically a poor thermal conductor.
Greater electrothermal efficiency can be achieved by integrating a microheater in the resonator structure. Such an approach also has potential drawbacks, however, because the integrated heater introduces parasitic current paths that can interfere with the operation of the device. This problem is especially severe in differentially driven modulators, in which a pair of complementary driving voltage signals are applied to the respective modulator contacts. If an integrated heater is present, charge will also flow out of the heater into one of the modulator contacts, and vice versa. The consequent extra charge in the resonator can shift the resonant wavelength in a direction opposite to the shift brought about by heating. Such partial cancellation of the thermo-optical effect reduces the efficiency of the heater.
Another reason why an integral heater may be disfavored is that it adds to the overall size of the device, which may be disadvantageous in applications subject to severe spatial constraints.
Accordingly, there remains a need for new approaches that achieve greater heater efficiency, particularly in compact configurations.